The invention relates generally to the electrical system of an automotive vehicle and, more particularly, to the voltage regulator circuits of such systems.
Present day automotive voltage regulators are essentially single purpose circuits employed to regulate the output voltage of the vehicle alternator to within a narrow band of voltages corresponding to the rated battery voltage, under varying load conditions. Simply stated, the regulator provides a modulation of the field current of the alternator as a function of existing battery voltage which is continuously sensed for maintaining the rated voltage level.
Systems have been developed which employ the vehicle alternators to supply relatively high power at above normal voltage to loads such as power tools and resistance heating wires embedded in window material for windshield deicing. To supply such high power, it is desirable to increase the alternator output voltage in order to keep the heating current from becoming excessive. This has been accomplished in one approach by temporarily disconnecting the standard automotive load and the regulator from the alternator, and having the battery supply the load current during the deicing period. In a more recently developed system, the resistance heating element is coupled in series between the alternator output and the battery and standard load. The regulator is controlled to regulate both a substantially elevated voltage at the alternator output and the voltage across the battery. The aforementioned system is disclosed in U.S. Pat. No. 4,267,433 granted May 12, 1981 and entitled "Electrically Heatable Windshield Heater Control Circuit", inventor W. H. Sahm III, filed concurrently with and assigned to the same assignee as that of the present application, now issued as U.S. Pat. No. 4,267,433 on May 12, 1981.
When the alternator voltage is substantially increased over its normal operating voltage, it is possible for the alternator field current to exceed its rated value. Since conventional regulators respond only to battery voltage, or to other output voltages as in the above referenced patent application, they do not protect against excessive field currents that may be generated. Thus, it would be desirable to have a regulator providing an added control of the alternator that is responsive to field current.